Promeshes are the detailed 3D models used for the environment (walls, ground, even water). They are located in Data\World\ProMeshes and are named like world textures are. Going off memory, they only appear in 1st and 2nd person view and only within a certain distance. If no promesh model is defined, it defaults to a single triangle (two make a wall, of course). This is easy to see with the Recharge Seam. Its ID number is 67, but there is no associated promesh. Get up close to one in 1st person and you'll see how it doesn't match up. Here are some CFG entries which may help you.
WallProMeshes TRUE
Textures {
Lava {
rooftexture World\WorldTextures\lavaroof.bmp
surftextwidth 8
surftextheight 8 meshbasename World\ProMeshes\LavaPM
texturebasename World\WorldTextures\LavaSplit\Lava
}